填充第 5 題
\(\Delta ABC\)中，\(\angle A,\angle B,\angle C\)之對邊為\(a,b,c\)三數成等差，\(\angle B=30^{\circ}\)，\(\Delta ABC\)面積為\(\displaystyle \frac{3}{2}\)，試求\(b=\) 。
[解答]
令 a = b - d，c = b + d
(1/2)ac * sin30 = 3/2
b^2 - d^2 = 6
b^2 = a^2 + c^2 - 2accos30
b^2 + 2d^2 = 6√3
b = 1 + √3
